Apollo 15 CSM in lunar orbit. The SIM Bay, a scientific instrument compartment built into the CSM's side, is visible near the center of the image. Image: NASA

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Artist concept of Apollo-Soyuz docking. The Apollo CSM carries on its nose the Docking Module, a new hardware element required to make the Apollo and Soyuz spacecraft compatible for docking. Image: NASA

Apollo Returns to Its Earth-Orbital Roots (1971)

View of the nose of the Apollo 9 CSM in Earth orbit. Image: NASA

The United States began to abandon the technology of manned lunar exploration by late 1967, nearly a year before the first Apollo astronauts reached space on board an Apollo spacecraft (Apollo 7, 11-22 October 1968). Abandonment of the moon began with deep cuts in the Apollo Applications Program (AAP), NASA’s planned successor to the Apollo moon program. Ambitious two-week stays on the moon were among the first AAP missions to feel the budget-cutters’ blades.

By late 1970, NASA was hard at work returning Apollo to its roots. From its conception in late 1959 until President John F. Kennedy’s 25 May 1961 call for a man on the moon by 1970, NASA had seen Apollo primarily as an Earth-orbital spacecraft. NASA intended to use Apollo in the second and third phases of its 1960s piloted space program. The first phase, characterized by brief sorties, would be accomplished using one-man Mercury capsules. In the second, three astronauts would live and work on board Apollo spacecraft for longer periods. They would use a pressurized Orbital Module attached to their spacecraft as a small space station. The third phase would see Apollo spacecraft deliver crews and supplies to separately launched Earth-orbiting space stations. An Apollo circumlunar mission – a flight around the moon without capture into lunar orbit – was considered likely no earlier than 1970.

Model of the unflown Convair Apollo spacecraft. Like those of its competitors, General Electric and Martin, Convair’s design was conceived mainly as an Earth-orbital spacecraft. Image: NASA

Following six-month studies, General Electric (GE), The Martin Company, and Convair in mid-May 1961 submitted Apollo spacecraft designs that accommodated NASA’s three-phase plan. In the event, none flew; after Apollo was promoted to the role of NASA’s lunar landing spacecraft, the agency funded new studies and selected North American Aviation (NAA) as its Apollo spacecraft contractor.

Initially, NASA intended to land NAA’s Apollo on the moon atop a descent stage with landing legs. In July 1962, however, the space agency opted for the Lunar Orbit Rendezvous (LOR) mode for its moon landing. NAA’s Apollo Command and Service Module (CSM) spacecraft became strictly a lunar-orbital spacecraft, and to Grumman’s bug-like Lunar Module (LM) went the honor of landing on the moon.

As flown, the CSM, which measured a little more than 11 meters long, comprised the conical Command Module (CM) and the drum-shaped Service Module (SM). The Orbital Module of the GE, Martin, and Convair designs was judged to be unnecessary for lunar landing missions. In fact, some perceived that the LM had replaced the Orbital Module. The CM’s nose carried a probe docking unit, and at the aft end of the SM was mounted the Service Propulsion System (SPS) main engine. Interestingly, the SPS remained sized for CSM launch from the lunar surface, which made the engine more powerful than was necessary for CSM insertion into and escape from lunar orbit.

The CM also included the pressurized crew compartment, crew couches, flight controls, rendezvous aids, a bowl-shaped heat shield for Earth atmosphere reentry, and parachutes for descent to a gentle splashdown at sea. The SM, which was discarded before atmosphere reentry, included hydrogen-oxygen fuel cells for making electricity and water, propellant tanks, four attitude-control thruster quads, heat radiators, a high-gain radio antenna, and room for a side-mounted Scientific Instrument Module (SIM) Bay.

A decade after Kennedy’s call, NASA was busy readying itself for Skylab A, its first Earth-orbiting space station. Skylab A would receive at least three Apollo CSMs bearing three-man crews. The agency also studied independent CSM missions in Earth orbit and CSM missions to Earth-orbiting stations other than Skylab A.

On 27 August 1971, Philip Culbertson, director of the Advanced Manned Missions Program at NASA Headquarters in Washington, DC, dispatched a letter to Rene Berglund, Manager of the Space Station Project Office at NASA’s Manned Spacecraft Center (MSC) in Houston, Texas. In it, he outlined five Earth-orbital CSM missions that were “still under active consideration” at NASA Headquarters. Culbertson explained that his letter was meant to “emphasize the importance” of statements he had made in a telephone conversation with Berglund on 19 August.

Culbertson referred to an unspecified new contract MSC had awarded to CSM contractor North American. He told Berglund that, in “the early stages of your contract. . .you should concentrate on defining the CSM modifications required to support each of the [five] missions and possibly more important defining the effort at North American which would hold open as many as possible of the [five] options until the end of the [Fiscal Year] 1973 budget cycle.” U.S. Federal Fiscal Year 1973 would end on 1 October 1973.

The first and simplest of the five missions was an “independent CSM mission for earth observations.” The mission would probably use a CSM with a SIM Bay fitted out with remote-sensing instruments and cameras. At the end of the mission, an astronaut would spacewalk to the SIM Bay to retrieve film for return to Earth in the CM.

Apollo 15 CSM in lunar orbit. The SIM Bay, a scientific instrument compartment built into the SM’s side, is visible just above the center of the image. Image: NASA

The second mission on Culbertson’s list was an Apollo space station flight unlike any envisioned in the year before Kennedy diverted Apollo to the moon. It would have seen a CSM dock in Earth orbit with a Soviet Salyut space station.

Salyut 1, the world’s first space station, had reached Earth orbit on 19 April 1971. The 15.8-meter-long station remained aloft as Culbertson wrote his letter, but had not been manned since the Soyuz 11 crew of Georgi Dobrovolski, Viktor Patsayev, and Vladislav Volkov had undocked on 29 June 1971, after nearly 24 days in space (a new world record). The three cosmonauts had suffocated during reentry when their capsule lost pressure, so the Soviet Union had halted manned missions while the Soyuz spacecraft underwent a significant redesign.

The third Earth-orbital CSM mission on Culbertson’s list combined the first two missions into a single mission. The CSM crew would turn SIM Bay instruments toward Earth before or after a visit to a Salyut.

Culbertson’s fourth CSM mission would see the Skylab A backup CSM (CSM-119) with a crew of three dock first with a Salyut for a brief time, then undock and rendezvous with the dormant Skylab A Orbital Workshop. After docking with 26-meter-long Skylab A, the CSM-119’s crew would live and work on board for an unspecified period. NASA planned that, during the three missions to Skylab A in the basic Skylab Program, CSM-119 would stand by as a rescue vehicle capable of carrying five astronauts (Commander, Pilot, and the three rescued Skylab A crewmen). The Salyut-Skylab A mission, which would include no rescue CSM, was planned to begin 18 months after Skylab A reached orbit, or about nine months after the third Skylab A mission returned to Earth.

The fifth and final Earth-orbital CSM mission was really two (or, possibly, three) CSM missions. A pair of “90 day” CSMs would dock with the Skylab B station while a rescue vehicle modified to carry five astronauts stood by. Beginning in 1969 (that is, at the same time it started Skylab A funding), NASA had funded assembly of Skylab B as a backup in case Skylab A failed. Culbertson gave no date for the Skylab B launch, which would have required one of the two Apollo Saturn V rockets made surplus by the September 1970 cancellation of the Apollo 15 and 19 missions (the Apollo 20 mission had been cancelled in January 1970 to make its Saturn V available to launch Skylab A).

Of the five missions Culbertson declared to be on the table in August 1971, not one flew. Skylab A, re-designated Skylab I (but more commonly called Skylab), reached orbit on 14 May 1973 on a two-stage Saturn V. It suffered damage during ascent, but NASA and its contractors pulled it back from the brink. In August 1973, with Skylab I functioning well in Earth-orbit, NASA began to mothball its backup. Several plans for putting Skylab B to use were floated in the 1973-1976 timeframe, but Space Shuttle development had funding priority, so NASA’s second space station wound up on display in the National Air and Space Museum.

The three CSM missions to Skylab spanned 25 May-22 June 1973, 28 July-25 September 1973, and 16 November 1973-8 February 1974, respectively. Leaks in attitude control thrusters on the second CSM to dock with Skylab caused NASA to ready CSM-119 for flight; the leaks stopped by themselves, however, so the rescue CSM remained earthbound.

Artist concept of Apollo-Soyuz docking. The Apollo CSM carries on its nose the Docking Module, a new hardware element required to make the Apollo and Soyuz spacecraft compatible for docking. Image: NASA

In early April 1972, shortly before finalizing its agreement with NASA to conduct a joint Apollo-Salyut mission, the Soviet Union declared the concept to be impractical and offered instead a docking with a Soyuz. At the superpower summit in Moscow on 24 May 1972, U.S. President Richard Nixon and Soviet Premier Alexei Kosygin signed the agreement creating the Apollo-Soyuz Test Project (ASTP).

Apollo CSM-111 was the ASTP prime spacecraft, while CSM-119 was refitted to serve as its backup. In the event, the backup was not needed. CSM-111, officially designated “Apollo” (but sometimes informally called Apollo 18), docked with Soyuz 19 on 17 July 1975. CSM-111 did not include a SIM Bay. The last CSM to fly in space undocked on 19 July and, after a period during which its crew performed experiments in the CM, returned from Earth orbit on 24 July 1975, six years to the day after Apollo 11, the first moon landing mission, returned safely to Earth.